Antenna systems have near-field and far-field radiation regions. The near-field is a region near an antenna where the angular field distribution depends upon the distance from the antenna. The near-field is generally within a small number of wavelengths from the antenna and is characterized by a high concentration of energy and energy storage in non-radiating fields. In contrast, the far-field is the region beyond the near-field, where the angular distributions of the fields are essentially independent of the distance from the antenna. Generally, the far-field region is established at a distance of greater than D2/λ from the antenna, where D is an overall dimension of the antenna that is large compared to wavelength λ, the wavelength of the radiating signal. Generally, antenna radiation or the radiating signal are considered to propagate in the far-field region as a plane wave.
Antennas used to create and exploit the energy in their near-field are found useful in RFID, nuclear magnetic resonance (NMR), quadrupole resonance (QR), resonant power transmission and other applications. Used in this manner, these antennas may commonly be referred to as sensor probes. For example, some Radio Frequency Identification (RFID) systems use near-fields for communications between the RFID transponder tag and the RFID interrogator, and the energy stored in the near-field may also provide the power to drive a microchip embedded in a passive RFID transponder tag. RFID systems are typically wireless, non-contact systems that use radio frequency electromagnetic fields to transfer information from an RFID card or transponder tag to a reader or interrogator for the purposes of automatic identification and/or tracking of the item to which the tag is attached.
At least some known explosive detectors and RFID systems use loop-type radiators (i.e., a loop-type transmitting antenna) for the interrogator or transmitting antenna, for example, an antenna consisting of a figure-eight shaped conductor, to reduce the creation of energy in their far-field regions. That is, loop antenna systems can be designed such that the coupling between the antenna and its nearby surroundings (i.e., the near-field) is relatively high, whereas the coupling between the antenna and its distant surroundings (i.e., the far-field) is minimized. Since the near-field energy is most important for sensor probes, such as the probe of the present invention, a minimal far-field energy is acceptable.
To minimize the far-field energy, two or more loops are used in combination, where the loops may have a specific size and geometry, such that the magnitude and direction of the current within the loops generate fields that cancel each other out in the far-field region (that is, the vector sum of the fields created from each of the antenna loops is close to zero.) But generally, as applied to the present invention, the structure of the far-field developed by energy emanating from the transmit antenna is of little concern and thus there is no need to suppress the transmission of the far-field energy.
In a reciprocal fashion, when using the loop antenna system as described above in a receiving mode, energy emanating from the far-field region induces voltages in the loops that are equal in magnitude, but opposite in polarity, such that they sum to zero at the output terminals of the receiving antenna, while the reception of near-field signals is little affected. This far-field suppression is a desirable feature in an antenna for use in sensor probes, such as explosive detectors and RFID systems.
One application for near-field sensor probes (including those using loop-type radiators as described above) is in a detection system used to exploit a material's Nuclear Quadrupole Resonance (NQR), where NQR is a radio frequency (RF) spectroscopic technique that serves as the basis for a system to detect and identify a wide range of materials based on detection of the resonances associated with their quadrupolar nuclei. The energy transmitted from a near-field probe excites this resonance in a material exhibiting this NQR resonance characteristic. The material then radiates a response signal, which must be detected by the probe's receiving antenna in the presence of radio frequency interference (RFI) in the environment and typical Gaussian noise in the receiver. The NQR response signal provides a unique signature of the material of interest that indicates the presence of quadrupolar nuclei in the radiated material. Exemplary uses for NQR detection include (but are not limited to), screening of airline baggage, parcel screening, detection of drugs/narcotics, and detection of explosives, such as detection of buried Improvised Explosives Devices.
One drawback with systems that use near-field probes and related technologies, especially for detection and screening of explosives, is the need to operate in the presence of significant RFI especially far-field RFI. Therefore, some means of suppressing this interference without significantly degrading near-field performance is required. Systems for suppressing far-field RFI are known and at least one is described above (far-field radiation that generates opposite-polarity voltages that cancel) but the components involved are subject to manufacturing variations and tolerances that limit their effectiveness.
Suppression of RFI is particularly relevant for NQR systems, because the responses are relatively weak signals in segments of the RF spectrum occupied by high-power radio stations and subject to significant man-made and atmospheric noise sources. Detection of NQR signals, using a near-field probe (antenna) such as a loop antenna, can be difficult in the presence of strong far-field noise sources/signals, such as AM radio transmitters, and nearby noise sources/signals, such as automobile ignitions, fluorescent lighting, computers, mobile phones, and other electronics devices.
The presence of strong far-field noise sources/signals presents a difficulty that arises at least in part because these kinds of noise sources can create substantial coherent and non-coherent geographically distributed noise that can be within the detection frequency ranges of interest. For example, detection of land mine explosives such as tri-nitro-toluene (TNT) can be affected by amplitude modulation (AM) radio signals sourced in the far-field, because the characteristic detectable frequencies associated with TNT (used in NQR detection systems) are below 1 MHz, which is within the standard AM radio band.
It is desirable to suppress RFI emanating from distant sources, so that this RFI does not interfere with detection of the desired signal. Some known implementations that attempt to suppress RFI use a single sensor probe to implement both transmit and receive functions, augmented with a remote RFI sampling antenna (which will sense all energy in its vicinity, but is conventionally placed at a location where it will be less sensitive to the desired target's response to near-field energy) coupled to a weighted negative feedback loop to cancel the RFI and thereby reduce probe susceptibility to RFI. Such a system is referred to as an active cancelation network. This kind of implementation can introduce undesirable performance compromises that can lead to performance degradations. In particular, the desire to maximize the efficiency of the receive function works in opposition to the desire to limit the time it takes for the transmit energy in the probe to dissipate after the transmit pulse has ended. That is, it is preferable to limit the coupling between the transmit and receive energy in the probe. In fact, overall system performance is further improved by separating components associated with the transmit and receive functions so that each can be optimized for its specific function.
Additionally, the effectiveness of a remotely located sampling antenna is limited because the distributed nature of the RFI causes signals that are acquired from locations different from the location of the sensing probe to vary significantly in ways that cannot be fully compensated for by adjusting the phase and amplitude of the signal derived from the sampling antenna.
Specifically, the response from the remote sampling antenna(s) does not exactly match the response derived from the sensor probe. Outside of a narrow frequency band, the responses will differ in one or both of amplitude and phase. Use of remote sampling antennas can also impose stringent linearity requirements on the active components of the probe system, that is, the first stage of amplification (e.g., a low-noise amplifier (LNA)) to assure the desired response signal is not lost because of saturation of the amplifier before the RF-interference cancelation processing stage.
Still other implementations may incorporate shielding over some or all of the probe in an attempt to reduce RFI; this is more common with larger resonant probes, and can result in bulky probe configurations. Further, such shielding is best suited for detection of buried threats, but is much less effective in personnel screening applications.
Several previously patented inventions (U.S. Pat. No. 8,717,242 B2, U.S. Pat. No. 7,714,791 B2) and a published patent application of a pending application (US 20150372395 A1) by the current inventor have described means of suppressing RFI using a set of properly connected and properly sized, collocated loops. Specifically, two or more smaller loops are collocated at the center of a larger loop such that the total area of the smaller loops equals the area of the larger loop. The smaller loops are connected to the larger loop such that the voltage induced in the smaller loops by any RF interference sourced at a distance, that is in the far-field, is equal and opposite in polarity to the voltage induced in the larger loop. Thus, the interconnection of the loops acts to minimize the voltage developed due to far-field radiation impinging on the small and large loops.
However, fields generated in the near proximity of the loops (near-field radiation) do not induce equal voltages in the various loops. Specifically, the voltage induced by fields generated by sources near the larger loop is significantly less than the voltage generated in the smaller loops. For this reason, this combination of loops is seen to be a very effective means of sampling fields nearby, while rejecting fields sourced from substantially greater distances.
But, to maximize rejection of the far-field sourced interference the geometry of the loops must be carefully and accurately controlled to maintain proper size and alignment conditions. Manufacturing tolerances and nearby environmental conditions can compromise the RFI suppression due to these geometrical anomalies. Prior art techniques have demonstrated various means, both mechanical and electrical to maintain geometrical features and thereby maximize the suppression effect. Mechanical approaches are best suited for onetime adjustments or require complicated electromechanical actuators to provide automated control. Previously described electrical techniques, while suitable for automation, are complicated to implement.